1. Field of the Invention
The present invention relates to an induction heating apparatus which is excited by a commercial frequency power source, especially induction heating cooking apparatus excited by a commercial frequency power source.
2. Description of the Prior Art
Heretofore, induction heating cooking apparatus excited by high frequency such as about 10 - 30 K Hz has been known. However, since it requires a high frequency power source, it is quite expensive.
An induction heating cooking apparatus excited by a low frequency commercial power source such as 50 - 60 Hz has been proposed by the inventors. However, this apparatus had low heating efficiency and/or generated noise. The inventors have developed an induction heating cooking apparatus excited by a commercial frequency power source using two important inventions. The patent applications covering these inventions will be designated below.
The first invention is to provide a double layer structure connecting a non-magnetic highly conductive plate to the lower surface of the ferromagnetic plate at the bottom of the cooking pot. FIG. 1 shows one embodiment of the cooking pot wherein the reference numeral 1 designates a ferromagnetic member such as iron; 2 designates a non-magnetic highly conductive member such as aluminum or copper; and 3 designates a cover.
The second invention is to provide two groups of excitation circuits for generating magnetic flux across a cooking pot which gives a 90.degree. phase difference whereby the alternating components of electromagnetic force applied to the cooking pot are cancelled in order to prevent the generation of noise. A practical induction heating cooking apparatus operated by a commercial frequency power source has been made possible by these two inventions.
In the cooking pot previously invented, the electrical heating efficiency (ratio of heat generated in the cooking pot to the electrical input) was about 91 percent. However the total heating efficiency (ratio of the heat of the heated material in the pot to the electrical input) was about 70 percent. Thus, heat corresponding to about 20 percent of the electrical input escaped from the cooking pot to the environment as heat loss. The heat loss is caused by the heat transmission from the bottom of the cooking pot to the cover plate of the range table and by the radiation from the side walls and cover of the cooking pot to air. The heat transmitted from the cooking pot to the range cover plate results in a rise in temperature of the cover plate.